Stem cells have a tremendous capacity for self-renewal and regeneration of damaged tissues. Although we have learned a great deal about how these processes are regulated from studies of stem cells in culture, we have remained largely in the dark about the dynamic behavior of stem cells in their native microenvironment. To begin to view and understand the mechanisms and interactions that make stem cells renew, differentiate, and regenerate after injury it is essential to develop strategies that allow imaging of these cells in the context of a living animal. We have recently utilized a combination of transgenic approaches and live animal confocal microscopy to develop a novel imaging strategy that allows us to visualize in real time how individual hematopoietic stem cells (HSCs) behave in vivo over extended periods of time. Using this strategy we now propose to define 1) the influence of the niche on asymmetric and symmetric division of HSCs in vivo 2) how the niche and the interaction of a stem cell with the niche changes during regeneration and 3) how this changes during oncogenic transformation. Dynamic high resolution viewing of living stem cells in their native environment will be a powerful new and exciting avenue to understand endogenous regulation of stem cell function and how this is altered in regeneration and cancer. PUBLIC HEALTH RELEVANCE: The research proposed here will allow for in vivo monitoring of the signals and niche components that contribute to stem cell self renewal, regeneration and transformation. Insights gained from these studies will allow a better understanding of the signals influencing stem cells and aid in the development of new stem cell based therapeutic approaches enhancing human hematopoietic repair and may uncover new targets for treating leukemias.